source: Dev/trunk/d3/src/layout/tree.js @ 76

// Node-link tree diagram using the Reingold-Tilford "tidy" algorithm
d3.layout.tree = function() {
  var hierarchy = d3.layout.hierarchy().sort(null).value(null),
      separation = d3_layout_treeSeparation,
      size = [1, 1]; // width, height

  function tree(d, i) {
    var nodes = hierarchy.call(this, d, i),
        root = nodes[0];

    function firstWalk(node, previousSibling) {
      var children = node.children,
          layout = node._tree;
      if (children) {
        var n = children.length,
            firstChild = children[0],
            previousChild,
            ancestor = firstChild,
            child,
            i = -1;
        while (++i < n) {
          child = children[i];
          firstWalk(child, previousChild);
          ancestor = apportion(child, previousChild, ancestor);
          previousChild = child;
        }
        d3_layout_treeShift(node);
        var midpoint = .5 * (firstChild._tree.prelim + child._tree.prelim);
        if (previousSibling) {
          layout.prelim = previousSibling._tree.prelim + separation(node, previousSibling);
          layout.mod = layout.prelim - midpoint;
        } else {
          layout.prelim = midpoint;
        }
      } else {
        if (previousSibling) {
          layout.prelim = previousSibling._tree.prelim + separation(node, previousSibling);
        }
      }
    }

    function secondWalk(node, x) {
      node.x = node._tree.prelim + x;
      var children = node.children;
      if (children) {
        var i = -1,
            n = children.length;
        x += node._tree.mod;
        while (++i < n) {
          secondWalk(children[i], x);
        }
      }
    }

    function apportion(node, previousSibling, ancestor) {
      if (previousSibling) {
        var vip = node,
            vop = node,
            vim = previousSibling,
            vom = node.parent.children[0],
            sip = vip._tree.mod,
            sop = vop._tree.mod,
            sim = vim._tree.mod,
            som = vom._tree.mod,
            shift;
        while (vim = d3_layout_treeRight(vim), vip = d3_layout_treeLeft(vip), vim && vip) {
          vom = d3_layout_treeLeft(vom);
          vop = d3_layout_treeRight(vop);
          vop._tree.ancestor = node;
          shift = vim._tree.prelim + sim - vip._tree.prelim - sip + separation(vim, vip);
          if (shift > 0) {
            d3_layout_treeMove(d3_layout_treeAncestor(vim, node, ancestor), node, shift);
            sip += shift;
            sop += shift;
          }
          sim += vim._tree.mod;
          sip += vip._tree.mod;
          som += vom._tree.mod;
          sop += vop._tree.mod;
        }
        if (vim && !d3_layout_treeRight(vop)) {
          vop._tree.thread = vim;
          vop._tree.mod += sim - sop;
        }
        if (vip && !d3_layout_treeLeft(vom)) {
          vom._tree.thread = vip;
          vom._tree.mod += sip - som;
          ancestor = node;
        }
      }
      return ancestor;
    }

    // Initialize temporary layout variables.
    d3_layout_treeVisitAfter(root, function(node, previousSibling) {
      node._tree = {
        ancestor: node,
        prelim: 0,
        mod: 0,
        change: 0,
        shift: 0,
        number: previousSibling ? previousSibling._tree.number + 1 : 0
      };
    });

    // Compute the layout using Buchheim et al.'s algorithm.
    firstWalk(root);
    secondWalk(root, -root._tree.prelim);

    // Compute the left-most, right-most, and depth-most nodes for extents.
    var left = d3_layout_treeSearch(root, d3_layout_treeLeftmost),
        right = d3_layout_treeSearch(root, d3_layout_treeRightmost),
        deep = d3_layout_treeSearch(root, d3_layout_treeDeepest),
        x0 = left.x - separation(left, right) / 2,
        x1 = right.x + separation(right, left) / 2,
        y1 = deep.depth || 1;

    // Clear temporary layout variables; transform x and y.
    d3_layout_treeVisitAfter(root, function(node) {
      node.x = (node.x - x0) / (x1 - x0) * size[0];
      node.y = node.depth / y1 * size[1];
      delete node._tree;
    });

    return nodes;
  }

  tree.separation = function(x) {
    if (!arguments.length) return separation;
    separation = x;
    return tree;
  };

  tree.size = function(x) {
    if (!arguments.length) return size;
    size = x;
    return tree;
  };

  return d3_layout_hierarchyRebind(tree, hierarchy);
};

function d3_layout_treeSeparation(a, b) {
  return a.parent == b.parent ? 1 : 2;
}

// function d3_layout_treeSeparationRadial(a, b) {
//   return (a.parent == b.parent ? 1 : 2) / a.depth;
// }

function d3_layout_treeLeft(node) {
  return node.children ? node.children[0] : node._tree.thread;
}

function d3_layout_treeRight(node) {
  return node.children ? node.children[node.children.length - 1] : node._tree.thread;
}

function d3_layout_treeSearch(node, compare) {
  var children = node.children;
  if (children) {
    var child,
        n = children.length,
        i = -1;
    while (++i < n) {
      if (compare(child = d3_layout_treeSearch(children[i], compare), node) > 0) {
        node = child;
      }
    }
  }
  return node;
}

function d3_layout_treeRightmost(a, b) {
  return a.x - b.x;
}

function d3_layout_treeLeftmost(a, b) {
  return b.x - a.x;
}

function d3_layout_treeDeepest(a, b) {
  return a.depth - b.depth;
}

function d3_layout_treeVisitAfter(node, callback) {
  function visit(node, previousSibling) {
    var children = node.children;
    if (children) {
      var child,
          previousChild = null,
          i = -1,
          n = children.length;
      while (++i < n) {
        child = children[i];
        visit(child, previousChild);
        previousChild = child;
      }
    }
    callback(node, previousSibling);
  }
  visit(node, null);
}

function d3_layout_treeShift(node) {
  var shift = 0,
      change = 0,
      children = node.children,
      i = children.length,
      child;
  while (--i >= 0) {
    child = children[i]._tree;
    child.prelim += shift;
    child.mod += shift;
    shift += child.shift + (change += child.change);
  }
}

function d3_layout_treeMove(ancestor, node, shift) {
  ancestor = ancestor._tree;
  node = node._tree;
  var change = shift / (node.number - ancestor.number);
  ancestor.change += change;
  node.change -= change;
  node.shift += shift;
  node.prelim += shift;
  node.mod += shift;
}

function d3_layout_treeAncestor(vim, node, ancestor) {
  return vim._tree.ancestor.parent == node.parent
      ? vim._tree.ancestor
      : ancestor;
}